This invention relates to improved compositions and processes for enhancing brightness and optical properties of a paper product in presence of optical brightening agents (OBA). The effect of the said compositions comes from a combination of brightening (post-bleaching) of the pulp and synergistic activation of optical brighteners.
Pulps produced by either mechanical or chemical pulping methods possess a color that can range from dark brown to creamish depending on the wood type and defibering process used. The pulp is bleached to produce white paper products for a multiplicity of applications. Bleaching is the removal or alteration of those light-absorbing substances found in unbleached pulp. In the bleaching of mechanical pulp, the object is to decolorize the pulp without solubilizing the lignin. Either reducing (e.g. sodium hydrosulfite) or oxidizing (e.g., hydrogen peroxide) bleaching agents are usually used. The bleaching of chemical pulps is an extension of the delignification that started in the digestion stage. The bleaching is often a multistage process, which stages may include chlorine dioxide bleaching, oxygen-alkaline delignification, and peroxide bleaching. Discoloration mostly ascribed to thermal aging, results in yellowing and brightness loss in various stages of papermaking processes employing bleached pulp and in the resultant paper products. The industry invests significantly in chemicals such as bleaching agents and optical brighteners that improve optical properties of the finished paper or paper products.
Peracetic acid (PAA) and its salts, optionally in a combination with hydrogen peroxide, were proposed as chemicals for pulp bleaching and delignification (see U.S. Pat. Nos. 6,258,207 and 5,785,812, US Published Patent Applications 2012/0120067532 and 2004/0000035537, International Patent Applications WO 0052258, WO 9932710, and WO 19990701, Jap. Pat. 06002283, and scientific articles: Environmental friendly pulping and bleaching of rapeseed stalk fibers, by Tofanica, B. M. et al., Environmental Engineering and Management Journal, Vol. 11 Issue 3, pp. 681-686, (2012), Effect of last stage bleaching with peracetic acid on brightness development and properties of eucalyptus pulp, by Barros, D. P. et al, BioResources, Vol. 5 Issue 2, pp. 0881-0898 (2010), Ramos, E.; Bleaching of soda pulp of fibers of Musa textilis nee (abaca) with peracetic acid, by Jimenez, L. et al, Bioresource Technology, Vol. 99 Issue 5, pp. 1474-1480, (2008), Bleaching of olive tree residues pulp with peracetic acid and comparative study with hydrogen peroxide, by Lopez, F., Industrial & Engineering Chemistry Research Vol. 41 Issue 15, pp. 3518-3525 (2002), The effect of peracetic acid treatment of bleached kraft pulp in fine paper production, by Jakara, J., Preprint—PAPTAC Annual Meeting, 87th, Montreal, QC, Canada, Book B B1-B6, Jan. 30-Feb. 1, 2001 (2001), Application of peracetic acid in chemical pulp bleaching, Tripathi, S., Ippta Journal, Vol. 19 Issue 1, pp. 77-82, (2007), and Delignification and bleaching with peracids. Part 1. Comparison with hydrogen peroxide, by Delagoutte, T., Paperi ja Puu, Vol. 81 Issue 7, pp. 506-510, (1999).
These references show that sometimes PAA may enhance the efficiency of fluorescent brightening agents. Also when applied under paper mill conditions, PAA may enhance the brightness of pigments and as a side effect efficiently prevent microbial growth in paper machines. In addition PAA may inhibit brightness reversion when PAA is applied to bleached kraft pulp in the bleached pulp storage chests of integrated pulp and paper mills (10-15% consistency), or in the stock preparation chests of paper machines (2-4% consistency). The findings in these references however were not conclusive. In some cases it was found that PAA application as the last pulp bleaching stage had no significant effect on pulp reversion and L*a*b* coordinates. Pulp bleaching applications, generally, require rather high concentrations of the reagent and long process time, e.g., 55C, 4.5% PAA and a bleaching time of at least 150 min. Use of PAA at a dose of 0.5-1.0 kg/TP at ambient temperature and normal plant pulp increased the final pulp brightness by 1.0-1.5 points. Post-bleaching with PAA can be carried out after the bleach plant steps in a pulp flow pipe, a storage tower or in the paper machine at a paper mill.
European Patent Application 1389646 (2004) proposed treatment of a pulp suspension (2-4% consistency) with a composition of peroxyacetic acid and hydrogen peroxide that made the pulp more receptive to the activities of OBA and therefore providing greater paper quality improvement relative to the relative volume of OBA added. It utilized a common commercial PAA that is a blend of approximately 15% PAA peroxyacetic acid and approximately 14% hydrogen peroxide.
A major limitation in all of these applications however is that they are not designed for introduction during a size press operation. Introduction during a size press operation allows for increased efficiencies it terms of both utilization of active components and time of the reaction (almost instantaneous). As a result there is ongoing need and clear utility in a novel improved method and/or composition and/or apparatus for improving brightness in pulp. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.